A typical Lithium-Ion (Li-Ion) battery pack, when used as a DC voltage power supply, usually includes a group of battery cells connected in series. When the group of cells connected in series are charged for a time period, voltages of some cells increase faster than voltages of other cells due to different characteristic of each cell. In one instance, a cell is unbalanced if the voltage of the cell is greater than a predetermined threshold. In another instance, a cell is unbalanced if a first voltage of this cell is greater than a second voltage of another cell and the voltage difference between the first voltage and the second voltage is greater than a predetermined threshold. Since voltages of some cells increase faster than voltages of other cells, at the end of a balancing cycle, the cells may have different cell voltages.
FIG. 1 illustrates a conventional charging circuit 100 for charging a battery pack including multiple cells 101, 102 and 103. The cells 101, 102 and 103 are connected in series with a switch 130. A charger 110 charges the cells 101, 102 and 103. During the charging operation, if any cell voltage reaches a predetermined threshold, the charger 110 turns off the switch 130 and the charging of cells 101, 102 and 103 are stopped. As the result, at the end of the balancing cycle, some cells are not fully charged.
FIG. 2 shows another conventional charging circuit 200 which utilizes multiple chargers 201, 202 and 203 to charge multiple cells 101, 102 and 103. The drawback for this charging circuit 200 is that the cost of the charging circuit 200 is increased because each cell has a charger.